A zero-voltage and zero-current switching three-level DC-DC converter with reduced rectifier voltage stress

A novel ZVZCS three-level converter with PWM phase-shift control is proposed. The ZCS of the inner leg is obtained by using a regenerative passive snubber in the secondary. In order to reduce the voltage stress on the rectifier´s diodes, a few passive elements are inserted into the primary: a small inductance, two diodes and a small additional winding of the main transformer. In each half-cycle, one of these diodes conduct for a short moment in order to clamp the voltage of the snubber´s capacitor at (n₂/n₁)(V_in/2), n₁ and n₂ being the primary, and respectively the secondary´s turns number. Thus, the voltage stress on the rectifier is clamped. The three-level configuration allows for the reduction of the voltage stress across the power switches to half of the input voltage, V_in. The zero-voltage-switching of the outer leg is realized in a large load range by using the energy stored in the output filter´s inductor. The regenerative snubber serves to resetting the primary current for achieving ZCS, and thus eliminates high circulating energy in the freewheeling stage. As a result of the above improvements, a high efficiency is obtained. The theoretical analysis and the performance expectations are confirmed by experiments carried on a 4.5 kW prototype.